The present invention relates to a dispersion-shifted single-mode optical fiber.
Single-mode optical fibers that are referred to as xe2x80x9cdispersion-shifted fibersxe2x80x9d (DSFs) are such that, at the transmission wavelength at which they are used (which wavelength is, in general, different from the wavelength of 1.3 xcexcm for which the dispersion of silica is substantially zero), the chromatic dispersion of the transmitted wave is substantially zero; i.e. the non-zero chromatic dispersion of silica is compensated (hence, the use of the term xe2x80x9cshiftedxe2x80x9d) by increasing the index difference xcex94n between the core of the fiber and the optical cladding. This index difference makes it possible to shift the wavelength for which the chromatic dispersion is zero. It is obtained by inserting dopants into the fiber while it is being manufactured, e.g. by a modified chemical vapor deposition (MCVD) process that is known per se, and that is not described in any more detail herein. A typical value for the index difference between the cladding and the core of the fiber is 24xc3x9710xe2x88x923. The increase in refractive index in silica can be obtained by using germanium as the dopant. The terms xe2x80x9ccladdingxe2x80x9d and xe2x80x9ccorexe2x80x9d are known to the person skilled in the art; conventionally, the xe2x80x9ccladdingxe2x80x9d is the portion that extends to a diameter of 125 xcexcm. The core corresponds to the portion in which about 70% of the light energy propagates.
Such single-mode fibers must also preferably have characteristics that correspond to the requirements both of cable-makers and of system designers: firstly they must have small mode diameters, and good xe2x80x9ccablabilityxe2x80x9d (i.e. suitability for being included in a cable), involving bending capacity of the fiber and low attenuation; and secondly they must have mode diameters that are as large as possible, large effective areas, and suitable values for the zero-dispersion wavelength xcex0. When such fibers are used for wavelength-division multiplexed (WDM) transmission systems, the constraints are even more stringent.
It would be preferable to use a fiber having a xcex0 value that is greater than or equal to 1565 nm, and advantageously that is greater than or equal to 1585 nm, in order to avoid using a dispersion-compensating fiber (DCF), which suffers from the drawbacks of facilitating non-linear effects, of having high attenuation, and of being difficult to lay in a cable. Unfortunately, existing fibers having such xcex0 values have small effective areas or high attenuation around 1550 nm.
Numerous index profiles have been proposed for such dispersion-shifted single-mode optical fibers. The index profile is generally described as a function of the appearance of the curve representing the refractive index as a function of the radius of the fiber. The index profile is thus said to be xe2x80x9csteppedxe2x80x9d, xe2x80x9ctrapezium-shapedxe2x80x9d or xe2x80x9ctriangularxe2x80x9d for curves representing the variation of refractive index as a function of radius that are respectively stepped, trapezium-shaped, or triangular. Such curves generally represent the ideal or reference profile of the fiber, it being possible for the constraints involved in manufacturing the fiber to give rise to a profile that is significantly different.
Early dispersion-shifted optical fibers were of the index-step, triangle, Gaussian, or xcex1 power type. The characteristics of those fibers are no longer considered to be sufficient, in particular because of their small mode diameters and of their sensitivity to bending, even though those fibers have attenuation values lower than 0.20 dB/km at 1550 nm. A new family of fibers has thus been developed: fibers whose index profiles are Gaussian with a pedestal, or xe2x80x9cdual shape corexe2x80x9d index profiles.
Thus, Patent Application EP-A-0 789 257 describes a fiber having an index profile that is Gaussian with a pedestal, i.e. that is a xe2x80x9cdual shape corexe2x80x9d index profile. The index profile of that fiber is shown in FIG. 1. That fiber has a mode diameter of 9 xcexcm, a xcex0 wavelength of about 1582 nm.
Patent Document U.S. Pat. No. 5,278,931 describes an optical fiber having improved sensitivity to bending, with a core region having a refractive index greater than the refractive index of the remainder of the fiber, and a small diffusion tail at the interface between the core and the cladding. That document suggests optionally providing a ring of higher index appropriately chosen to lower the cutoff wavelength or to obtain a fiber that is less sensitive to bending.
Those various known solutions do not make it possible to obtain a xcex0 wavelength value that is high or that can be chosen freely, while also conserving the characteristics suitable for good xe2x80x9ccablabilityxe2x80x9d.
The invention proposes a solution that makes it possible to maintain an effective area Aef that is large, typically above 70 xcexcm2, thereby limiting non-linear effects. The fiber of the invention also makes it possible to obtain attenuation that is low, preferably less than or equal to 0.2 dB/km. In addition, it makes it possible, for all these parameters, to obtain a zero chromatic dispersion wavelength xcex0 that is advantageously greater than or equal to 1585 nm; which makes it possible (if the fiber is used for WDM transmissions) to avoid or to limit four-wave mixing and the use of dispersion-compensating fibers.
In other words, the invention proposes a method of increasing the value xcex0 of the wavelength for which chromatic dispersion is zero in a dispersion-shifted single-mode optical fiber having cladding with a given index (ns), a fiber core with an index profile that is Gaussian with a pedestal, i.e. a xe2x80x9cdual shape corexe2x80x9d index profile, and a mode diameter that is advantageously greater than or equal to 8 xcexcm, including the addition in the index profile of the fiber core of an outer annular portion of index greater than the index of the cladding.
The invention thus provides a dispersion-shifted single-mode optical fiber having:
cladding of given refractive index; and
a fiber core having an index profile that is Gaussian with a pedestal, i.e. a xe2x80x9cdual shape corexe2x80x9d index profile;
said fiber being characterized in that the index profile of the fiber core has an outer ring of index greater than the index of the cladding.
Advantageously, the index of said ring is greater than or equal to the index of the pedestal, and preferably lies in the range 1xc3x9710xe2x88x923 to 6xc3x9710xe2x88x923.
In an embodiment, said ring has a thickness lying in the range 0.3 times the radius of the Gaussian portion of the core to 0.8 times said radius.
In an embodiment, the maximum index of the Gaussian portion of the core is generally greater than or equal to 9.5xc3x9710xe2x88x923.
Preferably, the index of said pedestal is greater than the index of the cladding.
In an embodiment, the index of said pedestal generally lies in the range 1xc3x9710xe2x88x923 to 3xc3x9710xe2x88x923.
In another embodiment, the index between said pedestal and said ring is less than or equal to the index of the cladding. Such an index generally lies in the range 0 to xe2x88x920.5xc3x9710xe2x88x923.
Advantageously, the portion of the fiber between said pedestal and said ring has a thickness lying in the range 0.3 times the radius of the Gaussian portion of the core to 0.8 times said radius.
Between said outer ring and the cladding, the fiber may also have an annular portion of index less than or equal to the index of said outer ring and the index of the cladding. Such an index generally lies in the range 0 to xe2x88x920.5xc3x9710xe2x88x923.
The fiber of the invention is generally and preferably such that the value of the wavelength for which the chromatic dispersion is zero is advantageously greater than or equal to 1565 nm, and preferably greater than or equal to 1585 nm.
Advantageously, said fiber has an effective area greater than or equal to 70 xcexcm2.
Preferably, it has attenuation less than 0.2 dB/km for a wavelength value of 1550 nm.
The invention thus proposes to add a ring to existing fiber profiles of the Gaussian-with-pedestal or xe2x80x9cdual shape corexe2x80x9d type. It makes it possible to conserve low attenuation, and large effective area, while obtaining xcex0 values that are high and that are typically greater than or equal to 1565 nm, and advantageously greater than or equal to 1585 nm.
The invention generally makes it possible to obtain for the fiber a xcex0 wavelength of at least 1565 nm and preferably at least 1585 nm, together with the following characteristics:
an effective area greater than or equal to 70 xcexcm2; and
attenuation of less than or equal to 0.2 dB/km for a wavelength value of 1550 nm.